shortest: cache disambiguation revset This makes it actually useful. In compared the time in my hg repo with 69.6k revisions and with a disambiguation revset of "not public()" that matches 563 visible revisions. I ran "time hg log -T '{shortest(node1,)}' -r 0:1000" (no revisions within the revset in that revision range). Before this patch, it took 57s and after it took 0.7s. Differential Revision: https://phab.mercurial-scm.org/D4039